I've been involved with two show stops.
I had to stop when we had a theatre wide power outage. Only the battery backup asile lights were working and the emergency lights. I came out on stage, as we had no power for the God Mic and asked everyone to remain calm, reassuring them that the power would return and we would resume soon... you guessed it, it never did, so fearing the emergency lights might start dieing, we cleared the theatre and sent everyone home.
The other was to replace an actor who was injured when a fight scene went wrong. Commuity theatre doesn't have the luxury of stand bys so when an actor broke his ankle jumping down and landign horribly wrong during a sword fight, we needed to replace him with a wide eyed and very nervous ASM who amazingly has never worked with me again

He accutally wasn't bad and he only had a scene or two left.